The discovery of the piezoelectric properties of certain polymers has been exploited by many individuals to produce acoustic to electric or electric to acoustic transducers and other devices with related principles of operation.
U.S. Pat. No. 3,982,143 to Tamura et al. disclose a piezoelectric electro-acoustic transducer which includes a piezoelectric diaphragm backed by a resilient material such as a polyurethane foam. U.S. Pat. No. 4,008,408 to Kodama, U.S. Pat. Nos. 3,973,150, 3,976,897, and 3,997,804 to Tamura et al., U.S. Pat. No. 4,024,355 to Takashasti and U.S. Pat. No. 4,045,695 to Itagaki et al. disclose improvements in this concept.
U.S. Pat. No. 3,792,204 to Murayama discloses a peripherally supported, curved diaphragm vibrated by both piezoelectric and electrostatic principles, of a shape which may be that of a resonance body of a string music instrument.
Piezoelectric transducers utilizing resilient diaphragm backings are subject to change in properties such as sensitivity or frequency response as the backing changes resiliency with time or due to changes in environmental conditions such as temperature or humidity.
"Molded Piezoelectric Transducers Using Polar Polymers", by Micheron and Lemmon in the Journal of the Acoustical Society of America, Volume 64, No. 6 (December, 1978), page 1720 discloses piezoelectric films which are self shaped, requiring no backing to impart curvature. While not having the disadvantages of resiliently backed diaphragm transducers outlined above, these transducer diaphragms are subject to collapse when handled in a rough manner.
"Piezoelectric and Pyroelectric Polymer Sensors" by Seymour Edelman in Report on Sensor Technology for Battlefield and Physical Security Application Mobility Equipment, Research and Development Command, Fort Belvoir, Va., July 1977, at page 209 indicates that "the thinness of the polymer sheet permits it to be used as the active material in a light weight noise-cancelling microphone which responds well to a nearby source such as a speaker's lips while minimizing the effect of ambient noise."
U.S. Pat. No. 3,168,934 to Wilson discloses a microphone which is noise cancelling as a result of an inlet port near a speaker's mouth and an inlet port away from a speaker's mouth being connected by ducts of equal lengths to opposite sides of a diaphragm. This structure generally results in noise cancellation over a limited frequency range and some loss of sensitivity.
"Piezoelectric Polymer Transducers for Dynamic Pressure Measurements," by DeReggi et al, National Bureau of Standard Publications NBSIR 76-1078, June, 1976 at page 5 and in Appendix D, p 34-38, discloses the use of silver bearing rubber paint to make contact to a metal plating on a piezoelectric polymer film, and the disadvantages of the technique.
U.S. Pat. No. 3,970,862 to Edelman, et al. discloses the use of a silver epoxy dot to make electrical contact with the "hot" or ungrounded metallized surface on a piezoelectric polymer film.
The theory of design and application of resonant cavities is well known. Helmholtz resonators have been used in a variety of acoustic devices. A simplified design theory was reported by Lord Rayleigh in Volume II of The Theory of Sound, published by Macmillan and Co. in 1896 and is referred to in Modern Acoustics by A. H. Davis, published by Macmillan Company in 1934 at page 119 et seq. and is also discussed in A Handbook of Sound, by A. B. Wood, published by Macmillan Company in 1955.
"Electroacoustic Transducers Using Piezoelectric Polyvinylidenefluoride Films", by Reinhard Lerch, in the Journal of the Acoustical Society of America, Volume 66, No. 4, (October, 1979) at page 952 discloses the results of the computation of the sensitivity and lowest frequency of resonance as a function of the radius of curvature of dome shaped diaphragms.